Suppliers ready-mix it for the summer

The heat sensitivity of readymix is a perennial concern for concrete suppliers in the Middle East. But, as Zoe Naylor discovers, there is an increasing range of additives available to contractors to improve its workability.

|~|121prod200.gif|~|Readymix suppliers will be racing to deliver their products this summer. The extreme Gulf heat speeds up the hydration reaction of concrete, putting pressure on readymix suppliers to ensure delay-free deliveries to construction sites.|~|Blistering Gulf summers mean that readymix concrete manufacturers are in a constant battle against the clock to deliver mixture from plant to site. High temperatures combined with Dubai’s infamous traffic jams mean careful coordination is needed to prevent concrete from spoiling before it reaches its final destination. The solution is a carefully blended mixture, often with a variety of additives, which not only keep the concrete in pristine condition during transportation but also help to improve its workability once on site and ready for pumping. “The summer temperature is a problem in this part of the world for readymix concrete suppliers, since the high temperature speeds up the hydration reaction of the cement, thus reducing the time available for transporting and placing the concrete,” says Arturo Rodríguez Jalili, technical director of Cemex. This can have a major impact on the way in which large-scale concrete pours are carried out, such as on the Ras Al Khor Creek crossing in Dubai. A total of 200,000m3 of concrete will be poured for the overall project, out of which 50,000m3 will be poured for the underpass section alone. “The biggest challenge we face is the thick concrete pours required for the bottom slab of the underpass, which is 2.5 m at the thickest point,” explains Bernard Patze, project manager at Besix. Since this can mean single pours of up to 2,000m3 in one go, care is taken to prevent the temperature of the concrete from rising too much. To counter this Besix pours the concrete at varying temperatures to control the overall temperature in the pour, which in turn helps to avoid cracking of the concrete. “The bottom portion of the slab is poured with concrete at 32 degrees C, then we continue with the central portion poured at 25 degrees C, and finally the top is again poured at 32 degrees C,” explains Patze. This helps to keep the difference in temperature between the core and the surface to a minimum. According to Patze, the different temperatures are achieved by adding ice, chilled water (at 4 degrees C) and additives at a controlled rate, to the concrete: “The additives we use are mainly superplasticisers,” he says. “These give a better workability to the concrete with a relatively low water content, resulting in a better concrete quality.” Vishal Sharma, operations manager at Conmix, says that it is vital to control the temperature of such large concrete pours: “In the case of mass concrete structures there will be an increase in the temperature during the hydration of the cement. “As the cement-water reaction is exothermic [releases heat energy] by nature, the temperature rise within a large concrete mass where the heat is not quickly dissipated i.e. the core of the structure, can be very high,” he adds. And if the temperature differential in the core and surface of the concrete exceeds 200C, there are chances of developing thermal cracks in the structures. When designing the concrete for such structures, Sharma says that factors such as choosing the right type of the cement, the use of pozzolanic materials such as fly ash and GGBS (ground granulated blast furnace slag) and the use of admixtures to control the placement temperature of concrete, need to be considered. “Water reducing and set-controlling admixtures can be used to keep the concrete plastic in massive blocks longer, so that successive layers can be placed and vibrated before the previous layer sets,” he explains. “By using admixtures the water content in the mix can also be reduced, and hence for the same water to cement ratio, the cement content in the mix will be reduced. Low cement content means less heat generation.” According to Sharma, during the placing of concrete it is important to keep the temperature of the fresh concrete as low as possible (in this region the placement temperature should be less than 32 degrees C). “This is achieved by using ice in the form of flakes together with the use of chilled water at the readymix batching plants,” he says. But before any pours can take place the readymix first has to run the gauntlet of plant to site, which means dealing with Dubai’s infamous traffic congestion. With summer temperatures topping 50 degrees C the last thing readymix suppliers want is for their cement-laden trucks to be held up in traffic, since this will affect the workability (measured in slump tests) of the concrete. In the Gulf winter, when the outside temperature is around 250C, a batch of concrete may be sent from the plant with a 180mm slump. By the time it has reached the site after an hour, for example, the slump could be 150mm. But after an hour on the road in summer, the same 180mm slump could be 120mm. The reaction has begun more quickly due to the higher temperature. “While designing a concrete mix for a particular site, the major factor (other then the technical properties) is the transportation time from the concrete batching plant to the site,” explains Sharma. In such cases, readymix additives can be used to modify the properties of fresh concrete by increasing its workability retention. This means the mix can be transported to the site, placed at the particular structure and compacted. “Most of the superplastisisers contain set-controlling compounds that increase the setting time of the concrete, allowing more time for transporting and placing,” says Jalili. A key aspect of the readymix is the water to cement ratio, which has a crucial impact on the overall strength of the concrete. Extra precautions need to be taken during the summer when water evaporation occurs much quicker and the reaction of the cement with the water starts earlier. This poses a challenge: if the water content in the cement is reduced, the workability is affected, but if more water is added, then the strength of the cement is weakened. Admixtures, however, can be used to increase the workability of cement without increasing water content, or decrease in the water content without changing the workability. “Retardants and superplasticers are formulated to give water reduction for the same amount of cement while at the same time giving higher workability for the concrete mixture,” says Jalili. But perhaps one of the biggest headaches for Dubai’s readymix suppliers is the time-sensitive delivery of the mix itself. Traffic conditions and far-away site locations must be taken into consideration. “All the concrete mixes are usually designed to allow at least three hours between batching and placing,” explains Jalili. But he adds that most of the specifications allow only two hours between batching and placing: “If the time when the truck reaches the site exceeds two hours, usually it gets rejected.” If this happens there is nothing for the driver to do but turn around and head back to the plant with the rejected batch. But what happens if the driver encounters problems on the return journey? “There will always be time for the truck to come back and dump the concrete in our dumping yard,” says Jalili. “But if the truck has a major break down, for example, and the concrete sets, then we use high pressure water to remove the concrete from the drum, and sometimes we break the concrete manually from inside.” A range of chemical admixtures is available to help prolong the workability of cement. These are designed to not only enhance the properties of concrete in the plastic and hardened states, but also increase the efficiency of cementitious materials and improve the economy of the concrete mixtures. As summertime fast approaches, the region’s readymix suppliers will no doubt be looking for new ways to beat the heat.||**||